1. Field of the Invention:
This invention relates to an addressing method for a multiplexable, bistable liquid crystal display of the type known from the U.S. Pat. No. 4,317,115 and to a display of the type known from the U.S. Pat. No. 4,272,162.
2. Description of the Prior Art
A bistable liquid crystal display differs from the usual liquid crystal display in that its brightness-voltage curve exhibits hysteresis behavior. This means that there are two possible optical states within a certain range of applied voltages. The liquid crystals used in such displays consist of, for example, a nematic liquid crystal doped with a chiral additive. Upon applying a voltage in the range mentioned above, a display element adopts either one of two optically-distinguishable states, e.g. dark or bright, depending upon which branch of the hysteresis loop that particular display element is on. This bistability effect is employed in the U.S. Pat. No. 4,317,115. The addressing scheme described in this patent employs a holding voltage where the display elements of a liquid crystal matrix display can adopt two different optical states (dark or bright). Certain elements can be transformed into the bright optical state by applying a writing voltage to them. The holding voltage is then again applied and these display elements stay in this (bright) optical state. The holding voltage is the root-mean-square value of the voltage difference between the multiplexing signals applied to the row and column electrodes corresponding to that display element. Because this holding voltage and the writing voltage must lie very close together for high multiplexing rates, the contrast ratio for this addressing method is not particularly good. For this reason the display area is subdivided into a scanning region, which comprises five lines, and a holding region, which comprises the lines already written and the new lines which are to be written. The display elements in the scanning region lie at a higher holding voltage then the display elements in the holding region. By means of a writing voltage whose amplitude is larger than the higher holding voltage, the selected display elements are transferred into the bright optical state. The non-selected display elements remain in the dark optical state.
Disregarding the technical difficulties that such a subdivision of the display introduces, this known process has the disadvantage that the display can only be written in one region at a time. This is a relatively slow process because the display elements of the scanning region must fully respond to the applied driving voltage before the next region can be written.
A second serious disadvantage of this known addressing scheme is that the display still does not have an optimum contrast ratio. Detailed investigations have shown that for certain holding voltages the optically bright state spontaneously transforms to the optically dark state after a period of time, and that this storage time is dependent on the value of the holding voltage. In particular, the storage time of the optically bright state decreases as the holding voltage is decreased. The display must therefore be continually refreshed, say every 10 seconds, even when no further changes are made on the displayed information. This refreshing process introduces undesirable flicker effects.
The display described in the U.S. Pat. No. 4,272,162 employs the White and Taylor effect (see, for example, U.S. Pat. No. 3,551,026). The display consists of two parallel, transparent substrate plates upon which are arranged electrode layers and orientation layers. The orientation of the liquid crystal is obtained through the oblique evaporation of SiO. The pretilt angle of the liquid crystal optic axis on the orienting layer is about 45.degree.. The liquid crystal that is filled into the display cell consists of a nematic liquid crystal mixture having positive dielectric anisotropy, a pleochroic dye and a chiral additive. A liquid crystal layer thickness of about 10 .mu.m and a chiral pitch of about 5.7 .mu.m is chosen in order to eliminate the so-called "storage effect" of this display. It follows from these values that the thickness-to-pitch ratio is about 1.75.
A liquid crystal display is described in J. Appl. Phys., Vol. 53, No. 12 (December 1982) pages 8599-8606 which has two crossed polarizers and which is based on the bistability effect. This display possesses substrate plates having orienting layers produced by the oblique evaporation of SiO. The pretilt angle of the liquid crystal molecules measured with respect to the normal to the substrate amounts to about 55.degree.. The liquid crystal layer has a thickness of either 15.2 .mu.m or 15.7 .mu.m. The liquid crystal consists of a cyano-biphenyl mixture with the cholesteric additive cholesteryl nonanoate. The pitch of the liquid crystal is about the same as the thickness of the layer. The total twist angle of the liquid crystal molecules in going from one substrate to the other amounts to 360.degree.. The display is transformed into the optically bright state by applying a voltage that lies above a certain critical value. This state slowly passes into the optically dark state, a process which is initiated by dust particles or other cell imperfectons, and then spreads over the entire display area. This effect causes the displayed information to fade away with time (see FIG. 5 in this article). The display must therefore be refreshed constantly at definite time intervals.
For this reason the display mentioned above should have a very precise plate separation, the most uniform orientation over the entire area of the plates that is practicable, and no dust particles or other nucleation centers inside the cell. Because this requirement means that the spacers are only permitted in the borders of the display cell, large-area displays are impractical, since very thick and extremely flat glass substrate plates would be required. It goes without saying that this would entail a very costly production technology.